88 



OHIO FOSSILS 



Fig. 205 



the last two genera; see under Ptychospirina for dis- 

 tinction between these three similar genera. In add- 

 ition, the growth lines in Naticopsis , when they are 

 preserved, are very fine and the shell is very thick. 

 Three species have been recorded from the Columbus 

 limestone. 



Platyceras has a distinctive shape which is 

 mainly due to the rapid increase in size of the whorls 

 (fig. 205). The external ornamentation varies all the 

 way from nearly smooth shells, with a single low keel 

 on the body whorl only, to forms with knobs and 

 spines which may be as long as half the diameter of the shell or more. The 

 lip is detached from the last whorl in most specimens. A large number of 

 species have been recorded from our Devonian. P. dumosum (fig. 205) of 

 the Columbus and Delaware is the commonest one. 



Porcellia and Pleuro- 



Fig. 206 



notu s are easily separated 

 from the other genera in 

 our Devonian because the 

 spire is sunken below the level of the whorls; 

 they are coiled in one plane. Porcellia has a 

 row of wart-like bumps on the top and bottom of 

 the whorl and the entire whorl is covered with 

 fine, crowded axial and spiral ridges which give 

 the surface the appearance of bead work. 

 Porcellia sciota is found in the Columbus and P. 

 hertzer i (fig. 206) in the Delaware. 



Pleuronotus is like Porcellia but its 



Fig. 209 



Fig. 208 



Fig. 210 



ornamentation is different. 



The whorls are squarish in 



cross section, even in internal 



molds, and the surface has no 



bead-like ornamentation; 



instead, there are two strong 



ridges on the upper and lower 



edges of the whorl and strong, 



distant axial ridges. P. decewi 



( Euomphalus of older publications) is found in the 



Columbus and Delaware (fig. 207). 



In Mourlonia ( Pleurotomaria in older publica- 

 tions) the spire is not sunken below the last whorl 

 and the base of the shell is rounded. M. lucina 

 (fig. 208) is found in the Columbus. 



Turbonopsi s ( Turbo in older publications) re- 

 sembles Palaeotrochu s but its base is not flattened, 

 the keel is higher on the whorl, and the upper part 

 of the whorl has a row of wrinkle-like bumps. 

 Turbonopsis shumardi (fig. 209) occurs in the 

 Columbus. 



Isonem a has unkeeled whorls and its base is flat ana sunken. L humile (fig. 210) is found 

 in the Columbus. 



